The present invention relates to fluid transport vessels such as rail tank cars adapted to transport hazardous liquids. It further relates to systems for eliminating the water hammer effect resulting from vessel wrecks and derailments and thereby protecting the vessel end walls from failing.
One of the concerns in the transportation of liquefied materials and particularly liquefied hazardous materials in tank cars and tank trailers is the failure of the vessel during collisions or derailments. One of the modes of failure of any pressure vessel is the so-called "water hammer" effect. Water hammer is a transient pressure peak developed by sudden deceleration of a mass of fluid. The pressure developed is a direct function of the vessel's change in velocity and, therefore, is directly proportional to the velocity of the liquid and inversely proportional to the time during which deceleration occurs. Since the velocity of pressure wave propagation is about four thousand feet per second, the maximum pressure head (in feet of fluid) developed is roughly equal to: a times D divided by g, or 125.times.D, where "a" is the velocity of pressure wave propagation in the system, "D" is the change in velocity in feet per second, and "g" is a dimensional constant of 32.17 (pounds) (feet)/(pound force) (seconds times seconds).
The peak value of the pressure developed is further reduced if the time of deceleration can be made greater than the time required for the pressure wave to travel from the point of stoppage (impact) to the point of reflection (the length of the vessel) and back. Since the velocity of wave propagation is about four thousand feet per second, this period is typically about one eightieth to one fiftieth of a second in a transport vessel. Limiting the speed of travel of such vessels is about the only way in which the initial velocity of the liquid can be reduced. However, changes in the vessel design can be made to extend the minimum time for deceleration or to absorb the pressure spike with a dramatic effect on the maximum pressure developed.
The primary cause of significant water hammer pressure in a transport vessel are head-on collisions of the vessel with immovable and inflexible objects, such as rock faces or concrete abutments, which impact the head of the vessel and thereby bring the vessel to a sudden stop. The fluid in the vessel continues to travel in the original direction and at the original speed for a very short period of time before the entire space at the head of vessel is filled with the relatively incompressible liquid. At this time, the kinetic energy of the moving liquid must be converted to pressure and dissipated thereby doing "work" on the walls of the vessel and on the liquid itself.
This pressure can as much as several thousand pounds per square inch in a tank car of a liquid, such as water, traveling at, for example, sixty miles per hour (eighty-eight feet per second). This is equal to about 5,000 psig, which is calculated as follows: 88.times.125=1,100 feet of head, or about 5,000 psig. This pressure can be enough to burst the end wall of the vessel. The peak pressure can be reduced by increasing the elasticity or compressibility of the system and by increasing the time allowed for deceleration.
The prior art has occasionally dealt with the water hammer problem in transportation vessels by increasing the wall thickness, and therefore its strength, sufficient to resist the water hammer pressures. Increasing the thickness of the walls is of limited practical value today, however, since at today's high transportation speeds extremely thick walls are required to resist pressures of several thousands pounds per square inch. The frequency of water hammer induced failures is fortunately low primarily because the probability of a wreck resulting in a nearly "instantaneous" stop of a massive pressure vessel is extremely low. Typically in such wrecks a considerable period of time, in fractions of seconds, is expended as the vessel crushes its way through soft rock, dirt or debris. This time can even extend to several seconds if the vessel rebounds, bounces, tumbles, slides or rolls during its deceleration. Another known design in light wall tanks, such as gasoline trailers, provides internal baffling to impede the flow the liquid from one end of the vessel to the other during controlled stops. This is shown for example in U.S. Pat. No. 4,251,005, whose entire contents are hereby incorporated by reference.
In piping systems, the water hammer problem is addressed in one of two basic ways. First, the minimum time for deceleration is extended by means of slow operating valves. Second, a means of absorbing or relieving the pressures developed is provided. This means can include placing pressure relief valves near the point of stoppage. Another means is by placing gas filled chambers or dampeners near the point of stoppage to expend the kinetic energy of the slowing liquid in compressing the gas of the dampener.
Additionally, containers, such as bumpers, bags or drums, filled with fluids, such as water, oil, gas or sand, are used by highway safety engineers to expend the kinetic energy of a highway collision over considerable distance, and therefore time, to mitigate the forces developed in the collision.